Longitudinal strain enhancement and bending deformations in piezoceramics.

Piezoelectric materials directly convert between electrical and mechanical energies. They are used as transducers in applications such as nano-positioning and ultrasound imaging. Improving the properties of these devices requires piezoelectric materials capable of delivering a large longitudinal strain on the application of an electric field. A large longitudinal strain of more than 1% is generally anticipated in suitably oriented single crystals of specific compositions of ferroelectric materials. Polycrystalline piezoceramics typically show a longitudinal strain of approximately 0.2-0.4%. We demonstrate that when the thickness of a polycrystalline piezoceramic is reduced to such an extent that a large fraction of the grains are in the triaxial-biaxal crossover regime, the domain-switching fraction increases considerably. If the positive and the negative surfaces of the piezoceramic respond to electric fields symmetrically, as in the classical PbZrTiO, a longitudinal strain of approximately 1% can be achieved in a 0.2 mm disc of the morphotropic phase boundary composition (a 300% increase from a thickness of 0.7 mm). We show that oxygen vacancies in piezoceramics cause asymmetrical switching at the positive and negative surfaces, which causes thin piezoceramics to bend. We expect these findings will encourage further engineering of these mechanisms across different piezoelectric material systems, opening new applications for electromechanical actuation.